The invention relates generally to polymer articles having modified surfaces, such as an essentially hydrophobic polymer article having a hydrophilic surface resulting from entropically-enhanced migration of a miscible, hydrophilic component to the surface of the article.
A membrane having a hydrophobic core and a hydrophilic surface component is provided as well.
Control of the surface chemistry of polymeric articles and compositions has technological relevance to a variety of commercially-important areas such as the medical devices industry, personal products, coatings, membranes, etc. Many polymeric articles and compositions that are useful in these areas are defined by a particular type of material because of economic considerations or mechanical requirements. For example, an essentially hydrophobic material might be used for structural reasons where it would be desirable to provide a different type of surface, for example a hydrophilic surface, on the article. Other examples involve imparting a chemical functionality to a surface such as a chelating functionality or other functionality that can selectively remove particular species from solution, or otherwise expose a desired chemical characteristic. While many techniques exist for modifying surface properties of polymers, many involve multi-step processes and/or do not result in thermodynamically or physically-stable incorporation of surface-modifying components.
It is often a goal in polymer chemistry to create a polymer article having a surface of high surface tension (surface energy) relative to the article as a whole, since higher surface tension typically corresponds to better wettability. However, in polymer blends including a higher surface energy component and a lower surface energy component the lower surface energy component (lower wettability component) tends to be present disproportionately at the surface since surface energy is characterized by inter-molecular attraction. That is, thermodynamic considerations result in the component with the higher inter-molecular attraction residing below the surface where it can be surrounded by a higher number of like molecules, while the lower surface energy component resides at the surface where a molecule is inherently surrounded by less like molecules. Techniques exist for creating polymeric materials having higher surface tension components at the surface, but a problem typically encountered with conventional methods is the tendency of the surface to reconstruct over time through chain reorientation where the lower surface tension component migrates to the surface of the polymer. (e.g., Wu, Supra; Garbassi, et al., Supra). Such reconstruction is consequently accompanied by an irrevocable loss of desired surface properties.
The control of surface properties of acrylate polymers has technological relevance to areas including biomedical devices, latex paints and other coatings, textiles, and recording media. However, conventional techniques for modification of acrylate polymer surface chemistry typically is achieved through kinetically-governed processes that allow little control over the final surface composition and structure. Plasma and flame treatments, commonly employed to oxygenate surfaces in order to improve wetting and/or adhesion, invoke reaction cascades of bond scission, fragmentation, and crosslinking, yielding poorly-defined surface compositions. Chemical oxidation by acid treatment typically causes pitting and solubilization that modifies surface morphology in an uncontrolled fashion (E.g., Wu, Polymer Interface and Adhesion (Marcel Dekker, Inc., New York, 1982); Garbassi, et al., Polymer Surfaces: From Physics to Technology (John Wiley and Sons, West Sussex, 1994)). Grafting methods used to bond hydrophilic species like heparin or poly(ethylene glycol) to surfaces in order to improve biocompatibility typically yield low surface coverages (E.g., Pekna, et al., Biomaterials, 14, 189 (1993); Harris, J. M., ed., Poly(ethylene glycol) Chemistry: Biotechnical and Biomedical Applications (Plenum Press, New York, 1992)).
An alternative method of preparing a hydrophilic surface on a hydrophobic polymer article might be through the addition of a hydrophilic species to the polymer which selectively segregates to the surface upon processing, providing the desired surface hydrophilicity. This approach would be particularly useful if the hydrophilic additive were miscible with the polymer, so as not to adversely influence the bulk properties of the article, such as mechanical behavior or optical clarity. One such candidate additive might be poly(ethylene oxide), PEO, because of its high degree of hydrophilicity and well-known resistance to protein adsorption. PEO is known to be miscible in poly(methyl methacrylate) up to very high concentrations. It is also known, however, that the surface tension of PEO is somewhat higher than that of PMMA. From this, we would assume that a surface of an article prepared from a PMMA/PEO blend should be depleted with PEO, in order to reduce the surface energy. It has been reported that neither component is enhanced at the surface of such blends (Sakellariu, Polymer, 34, 3408, (1993)). However, in this study samples were annealed for only three hours at 170 C.
Membrane technology presents a particularly interesting challenge in connection with surface functionalization. The use of polymer membranes for water treatment has become increasingly widespread in the past thirty years in such applications as desalination of sea and brackish water, water softening, production of ultrapure water, and purification of industrial wastewater. Membrane processes have additionally been used to generate ultrapure water sources for the electronics and pharmaceutical industries, and to treat wastewater from such diverse industries as textiles and laundry, electroplating and metal finishing, petroleum and petrochemical, food and beverage, and pulp and paper.
Membrane processes offer significant advantages over conventional water treatment technologies. They require no phase change and are thus inherently less energy-intensive than distillation methods used for desalination. They provide an absolute filter for pollutants above a given pore size, and are hence more reliable than flocculation methods that can leave residuals in treated water if improperly performed. In addition, the modular and compact design of membrane filtration units offers great flexibility in the scale of operation. And because membranes can separate pollutants without chemical alteration, they allow for more cost-effective recovery of valuable components from wastewater.
However, membrane technologies suffer from critical materials-related drawbacks that limit their efficiency and lifetime, and hence cost-effectiveness in water treatment applications. In particular, membrane fouling is a major problem which results in reduced efficiency due to flux decline, high cleaning and maintenance costs, and low membrane lifetimes. The cleaning and replacement costs for ultrafiltration processes are estimated to account for 24% and 23%, respectively, of the total process costs. While careful system operation and flow-pattern design can reduce fouling by suspended particulates or precipitated salts, the adsorption of proteins onto membrane surfaces is more insidious, generating a monolayer film that provides a foothold for slower deposition processes which deteriorate membrane performance and lifetime substantially. Membranes used in reverse osmosis processes have additional materials-related limitations. While the cellulose acetate-based membranes most commonly found in this application exhibit high flux and good salt rejection, these polymers hydrolyze over time, generating physical holes in the membrane which reduces its useful lifetime. Clearly, the need exists for new membrane materials with improved fouling resistance and longer service lifetimes. Moreover, membranes with improved selectivity are sought for more cost-effective recovery of wastewater constituents.
Methods to impart hydrophilicity to hydrophobic membrane surfaces have primarily focused on the grafting or coating of hydrophilic species directly onto membranes. In general, this approach suffers from several drawbacks: 1) achievable grafting densities are typically low due to kinetic limitations, 2) grafting reactions require an additional processing step and are difficult to scale up, 3) grated monolayers are susceptible to wear or removal during membrane cleaning procedures. An appealing alternative approach which might circumvent these problems is the addition of a hydrophilic macromolecular component to the membrane material that selectively segregates to the membrane surface during processing. Membrane materials prepared by this approach can offer important performance and processing advantages over commercial membrane materials as well as coated and graft-modified membranes. Unlike typical coated membranes, the surfaces of these membranes present an additive which is intimately entangled with the matrix. Furthermore, where segregation can be accomplished through a thermodynamic driving force, xe2x80x9cself-healingxe2x80x9d membranes are possible, whereby surface-active additive material removed from the membrane surface can be replaced by further segregation of the branched component, optionally during a periodic annealing operation. Finally, surface localization of the branched component can occur during a standard processing step, thus eliminating the need for additional membrane fabrication steps.
A variety of surface-modification techniques have been described in the patent literature. For example, Varady, et al., in U.S. Pat. No. 5,030,352, describes modification of a hydrophobic chromatography solid phase with a block copolymer including hydrophobic domains and hydrophilic domains. The hydrophobic domains associate with the solid phase via hydrophobic-hydrophobic interaction, and the hydrophilic domains extend outwardly away from the surface. The technique involves the step of crosslinking the block copolymer in place to produce a hydrophilic surface coating masking hydrophobic regions of the solid phase.
Stedronsky, in U.S. Pat. No. 5,098,569, describes a surface-modified membrane including a modifying polymer adsorbed onto a surface of the membrane and uniformly crosslinked thereon.
Nohr, et al., in U.S. Pat. Nos. 4,923,914, 5,120,888, 5,344,862, 5,494,855, and 5,057,262, describe thermoplastic compositions designed to expose a particular desired surface characteristic. Typically, Nohr, et al. employ a hydrophilic additive that is immiscible (incompatible) with the bulk polymeric component under ambient conditions, and therefore is driven to the surface of the blend upon solidification due to this incompatibility (via enthalpy). In U.S. Pat. No. 5,494,855, Nohr, et al. described blends including additives having good tensile properties or surface wettability. Formulation of a blend having good surface wettability involves an additive having a molecular weight of as low as from about 350 to about 1,200. Low molecular weight additives typically migrate more readily in blends and articles, thus it would not be unreasonable to assume that in this patent there is a teaching that advantageous mechanical properties resulting from a higher molecular weight additive and advantageous surface properties resulting from migration of a lower molecular weight additive are mutually exclusive. Nohr, et al. use fumed silica to aid segregation.
U.S. Pat. No. 4,698,388 (Ohmura, et al.) describes a block copolymer additive for modifying the surface of polymeric material. The block copolymer includes a matrix-compatible portion and a portion having a characteristic desirably present at the surface which is incompatible with the matrix. Due to the incompatibility of the surface-modifying portion of the block copolymer, that portion is segregated to the surface while the compatible portion interacts with the polymer matrix to retain the additive in the matrix. U.S. Pat. No. 4,578,414 (Sawyer, et al.) describes fine denier, wettable fibers and/or filaments prepared from olefin polymers including a relatively short, polymeric wetting agent including a hydrophilic domain and a hydrophobic domain. The additive segregates such that the hydrophilic domain modifies the surface.
Allegrezza, et al., in U.S. Pat. Nos. 5,079,272 and 5,158,721, describe a porous membrane defined by an interpenetrating polymer network of a hydrophobic polymer and an in-situ-crosslinked, interpenetrating hydrophilic polymer. The described technique includes the step of annealing the network, whereby the hydrophobic component crystallizes, xe2x80x9cexcludingxe2x80x9d the hydrophilic component to the surface.
U.S. Pat. No. 5,190,989 (Himori) describes an AB-type block copolymer having a hydrophilic group and a group having an affinity for a resin. The block copolymer is oriented with the hydrophilic component toward the surface or interface of the resin.
Meirowitz, et al., in U.S. Pat. No. 5,258,221, describe a two-step process in which a surface of a hydrophobic polyolefin article is modified by contacting the surface with a copolymeric material above the glass transition temperature of the polyolefin to fuse the copolymeric material to the polyolefin. The copolymeric material includes a hydrophobic moiety compatible with the polyolefin and a modifying moiety (e.g. hydrophilic) incompatible with the polyolefin.
U.S. Pat. No. 5,328,951 (Gardiner) describes a technique for increasing the surface energy of an organic polymeric article, in particular a polyolefin article, by forming a blend including a base polymer and an amphiphile having a molecular weight of from about 150 to about 500 Daltons. The amphiphile has a lipophilic component compatible with the base polymeric material, which is thought to anchor the amphiphile in the base polymer, and a hydrophilic component less compatible with the polymeric base which resides at the surface of the article.
Membranes from miscible blends of PVDF with from 5% to 34% poly(methyl methacrylate) (PMMA) are reported by Nunes, et al., xe2x80x9cUltrafiltration Membranes From PVDF/PMMA Blendsxe2x80x9d, J. Memb. Sci., 73, 25-35, 1992; Ito, et al., xe2x80x9cpH-Sensitive Gating by Conformational Change of a Polypeptide Brush Grafted onto a Porous Polymer Membranexe2x80x9d, J. Am. Chem. Soc., 119, 1619-1623 (1997) describe graft-polymerization of benzyl glutamate NCA onto a porous PTFE membrane, and a study of the effects of pH and ionic strength on permeation rate. The rate of water permeation through the membrane was found to be slow under high-pH conditions and fast under low-pH conditions since, under high-pH conditions, randomly coiled graft chains extended to close the pores. Kojima, et al., xe2x80x9cSelective Permeation of Metal Ions Through Cation Exchange Membrane Carrying N-(8-quinolyl)-sulfonamide as a Chelating Ligandxe2x80x9d, Journal of Membrane Science, 102, 49-54 (1995) describe chemical attachment of a chelating reagent, selective for Cu2+ over Fe3+, to side chains of a polymer to create a cation exchange membrane. This polymer was diluted in a solvent and impregnated into a porous Teflon(trademark) PTFE membrane and the solvent was evaporated. Mika, et al., xe2x80x9cA New Class of Polyelectrolyte-Filled Microfiltration Membranes with Environmentally Controlled Porosityxe2x80x9d, Journal of Membrane Science, 108, 37-56 (1995) describe grafting of 4-vinylpyridine onto polyethylene and polypropylene microfiltration membranes. Grafting is UV-induced and results in membranes showing a pH valve effect and the capability of rejecting small inorganic ions in the presence of reverse osmosis.
Iwata, et al. (xe2x80x9cPreparation and Properties of Novel Environmental-Sensitive Membranes Prepared by Graft Polymerization Onto a Porous Membranexe2x80x9d, J. Memb. Sci., 38, 185-199, 1988) report a glow discharge technique to graft polyacrylamide and polyacrylic acid chains onto polyvinylidene fluoride (PVDF) membrane. The permeation rates and separation characteristics of membranes so treated were found to vary significantly with pH and ionic strength of the feed solution, both of which influence the configurations of the grafted chains. Variations in the pH and ionic strength of the feed solution vary the extent to which electrostatic forces between the charges along the grafted polyion chains are screened. At low pH, the negative charges along the grafted chains are heavily screened by positive counterions, and the chains adopt random coil like configurations. At high pH, the grafted chains are dissociated, and they adopt extended configurations due to electrostatic repulsion between the negative charges spaced along them, effectively blocking the pores. Addition of methanol (a poor solvent for PAAm and PAA) was shown to be another method of collapsing the grafted chains. While significant, the variations in permeation rate were not as pronounced as those demonstrated in the system of Ito, et al, which were probably emphasized by the helix-coil transition which occurs in that system. Hautojarvi, et al, (J. Memb. Sci., 108, 37, 1995) published a similar study of PVDF membranes graft-modified with poly(acrylic acid).
In many prior techniques for modifying surfaces, durability of the modified surface and/or physical or optical characteristics of the article may be compromised. In particular, where a surface-modifying component is water-soluble, the component can become disassociated from the polymer surface over time if the article is used in an aqueous environment and the surface-modifying component is not securely associated with the article. Polymer blends that exploit the incompatibility of a surface-modifying component run the risk of formation of micelles or other segregated groupings within the polymer, which can render a polymer opaque (disadvantageous in many circumstances). Since incompatibility is the property necessary for segregation in many techniques, these techniques inherently carry these potential drawbacks.
The academic literature describes studies involving surface migration of components of a polymer blend based upon their architecture. For example, Steiner, et al., Science, 258, 1126 25 (1992) and Sikka, et al., Phys. Rev. Lett., 70, 307 (1993), describe experiments on polyolefin blends demonstrating that, where components of the blends are similar in energy, more highly-branched components tend to segregate to the surface of the article. However, there is some controversy in the literature in that Steiner, et al. (Supra) report that it is not clear that surface migration of the more highly-branched polyolefin occurs due to its architecture. Indeed, in these systems since the more branched component is the lower surface tension component the more branched component would be expected to reside at the surface according to the reported technique.
Accordingly, it is an object of the present invention to provide a simple, inexpensive technique for generating thermodynamically-stable polymeric articles having a desired surface property. In particular, it is an object to provide a technique for generating thermodynamically-stable, relatively high-surface-energy surfaces on polymeric articles for a variety of purposes. It is another object of the invention to provide stable hydrophilic surfaces on various acrylate polymers to improve emulsification in latex paints, impart resistance to static charge build-up on compact discs and textiles, improve anti-fouling properties of intraocular lenses and dental composites, and increase the wettability of acrylates to inks, glues, and paints. It is another object of the invention to provide straightforward techniques for creation of membranes of a variety of polymers having desired surface properties, and robust membranes having desired exposed functionality.
The present invention provides a technique for imparting, to a surface of a polymeric article, a desired chemical functionality that differs from the article as a whole. The technique makes use of the discovery that, in a compatible blend of different polymeric components, migration of the more highly-branched component can be enhanced by entropy. The molecule having the greatest number of chain ends can be present at the surface with the least configurational entropy penalty.
Also provided is a technique for imparting a desired chemical functionality to a surface of a polymeric article involving subjecting a blend of at least of a first and a second polymer component to phase inversion and allowing the second component to migrate, disproportionately, to the surface of the blend. The phase inversion technique can be driven completely by enthalpy, or by entropy, or a combination. That is, the above-noted technique involving allowing migration of a more highly-branched component to a surface of a blend can be combined with a phase inversion technique.
In one embodiment the invention provides an article having a surface, comprising an entangled blend of a first, relatively lower-cohesive-energy polymer component and a second, relatively higher-cohesive-energy polymer component. The first and second polymer components are compatible with each other at room temperature, that is, are miscible. The second polymer component is present at the surface of the article in a ratio to the first polymer that is greater than the overall ratio in the article of the second polymer component to the first polymer component.
In another embodiment the invention provides an article, having a surface, comprising an entangled blend of a first, essentially hydrophobic polymer component and a second polymer component that is a random co-polymer entangled with the first polymer component. The second polymer component is more hydrophilic than the first polymer component. The second polymer component has a molecular weight of at least about 15,000 and is present at the surface of the article in a ratio to the first polymer component that is greater than the overall ratio in the article of the second polymer component to the first polymer component. The second polymer component can be a random co-polymer.
Also provided is an article having a surface, comprising an entangled blend of a first polymer having an affinity to water and a second polymer having an affinity to water. The first and second polymers are compatible at room temperature. The surface of the article has an affinity to water that is greater than the average water affinity of the total of the first and second polymers in the article.
All of the articles of the invention can be porous membranes having a desired surface chemical functionality.
In another aspect the invention provides a series of methods. In one embodiment a method involves providing a miscible blend of at least first and second polymer components each insoluble in water. The first component is essentially hydrophobic and the second component is more hydrophilic than the first component. The components are allowed to phase segregate to form a porous membrane having a core and a surface of greater hydrophilicity than the core.
In another embodiment, a method involves providing a fluid blend of a first, relatively lower-cohesive-energy polymer component and second, relatively higher-cohesive-energy polymer component that is compatible with the first polymer component at room temperature. The blend is allowed to harden to form a polymeric article having a surface. The second polymer component is present at the surface of the article in a ratio to the first polymer component that is greater than the overall ratio in the article of the second polymer component to the first polymer component.
In another embodiment, a method is provided in which a polymer membrane is fabricated having a particular surface chemical functionality. A polymeric fluid is provided that includes a blend of a first polymer component and a second polymer component that is compatible with the first polymer component at room temperature that includes a particular chemical functionality. The polymeric fluid is subjected to phase inversion and an article is recovered that includes the blend of the first and second polymers. The second polymer is present at the surface of the article in a ratio to the first polymer that is greater than the overall ratio in the article of the second polymer to the first polymer.
In another embodiment a method of making a polymer membrane is provided that involves providing a polymeric fluid including a blend of a first polymer component and a second polymer component compatible with the first polymer component at room temperature. An emulsion is formed by exposing the polymeric fluid to a fluid incompatible with the first and second components and allowing the incompatible fluid to form the emulsion in the polymeric fluid. A porous article is recovered from the mixture that includes a blend of the first and second polymers with the second polymer present at the surface of the article in a ratio to the first polymer that is greater than the overall ratio in the article of the second polymer to the first polymer.
In some embodiments the second polymer component is more branched than is the first polymer component. In all cases the second and first polymer components can have different functionalities, with the surface enhancement of the second polymer component allowing formation of a polymeric article, such as a membrane, having a desired surface chemical functionality. The first and second components can be thermodynamically compatible at room and use temperatures, in addition to being compatible as a melt, therefore a thermodynamically stable, surface-segregated article results. In one embodiment, each component has a molecular weight of at least about 5,000, and a very well-entangled combination of first and second components results in the article.
Any of the articles described herein can be acrylic, for example the above-described polymers can be acrylic polymers. One set of methods involves blending a first, essentially hydrophobic acrylic polymer with a second, more-hydrophilic acrylic polymer and allowing the more-hydrophilic polymer to be driven to the surface of the article. In another method, a fluid blend of an essentially hydrophobic acrylic polymer and a more-hydrophilic acrylic polymer is provided, the two polymers being compatible. The blend is hardened to form an article in which the more-hydrophilic acrylic polymer is present at the surface disproportionately. The same method can be carried out where the first and second acrylic polymers are not necessarily hydrophobic and more-hydrophilic, but differ in chemical functionality of another type.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure.